1 Top-of-Line Corrosion Prediction in Offshore Pipelines By Mohamad Mounes Sadek Interim Report submitted in partial fulfillment in the requirement for the Bachelor of Engineering (Hons) (Civil Engineering) MAY 2013 Universiti Teknologi Petronas Bandar Seri Iskandar 31750 Tronoh Perak DarulRidzuan
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Top-of-Line Corrosion Prediction in Offshore Pipelines
By
Mohamad Mounes Sadek
Interim Report submitted in partial fulfillment in the
requirement for the
Bachelor of Engineering (Hons)
(Civil Engineering)
MAY 2013
Universiti Teknologi Petronas
Bandar Seri Iskandar
31750 Tronoh
Perak DarulRidzuan
2
CERTIFICATION OF APPROVAL
Top-of-Line Corrosion Prediction in Offshore Pipelines
By
Mohamad Mounes Sadek
A project dissertation submitted to the
Civil Engineering Programme
Universiti Teknologi PETRONAS
in partial fulfillment of the requirement for the
BACHELOR OF ENGINEERING (Hons)
(CIVIL ENGINEERING)
Approved by,
________________________
(Dr. Zahiraniza Binti Mustaffa)
UNIVERSITI TEKNOLOGI PETRONAS
TRONOH, PERAK
May 2013
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CERTIFICATION OF ORIGINALITY
This is to certify that I am responsible for the work submitted in this project, that the
original work is my own except as specified in the reference and acknowledgements,
and that the original work contained herein have not been undertaken or done by
unspecified sources or persons.
____________________________
MOHAMAD MOUNES SADEK
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ACKNOWLEDGMENT
First and foremost, I thank and praise God that blesses to finish my task for this project
successfully.
I would like to express my utmost appreciation to my supervisor, Dr. Zahiraniza Binti Mustaffa
for her guidance, support and encouragement in order to accomplish the tasks. Being his
supervisee, it has been a pleasure.
Special thanks are due to the Civil Engineering Department at Universiti Teknologi Petronas
that offer facilities to work and study this research. Thanks to Mrs. Aishah for providing support
to complete the project.
Lastly, I would give thanks to all postgraduates and friends who assist me to understand
extremely in this field of study. Their motivation for learning is greatly appreciated.
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ABSTRACT
Pipeline system is the most efficient transportation of oil and gas in offshore
industries. Pipeline as most of other materials is subjected to deterioration over time.
The oil and gas industries elevated the concern about pipeline corrosion due to
itsnegative results on the efficiency of the pipeline system. The corrosion of pipeline
most probably will lead to an environmental issues consequent of materials leakage.
The materials inside the pipe tend to react with the pipeline wall leading to serious
corrosion and potential leakage. The corrosion takes a place all around the pipe wall.
Therefore, the corrosion in pipeline is divided to top of line corrosion (TLC) and
bottom of line corrosion (BLC) regarding its location. A study on Top of Line
Corrosion Prediction is presented in this research in order avoid system leakage and
prevent a future pipe failure.
The research result showed a development of corrosion over the years on pipe
line. The probability of future corrosion and the range of corrosion in the pipe were
studied and prediction equations are modeled. The model show the percentage of
corrosion depth in top of line and a comparison between a pipe line historical data
was established. The comparison illustrates how the corrosion increases over the
years and present its severity. The corrosion orientation model prediction will assist
the pipe line inspector to locate the extreme corrosion orientation in the pipe and
prevent the further decay.
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List of Figures
Figure 1. SEM image of a cross section of a steel specimen including an iron carbonate
scale[Adapted from fromNesic and Lee (2003)] ………………….…………………………… 12
Figure 2. Influnce of temperature on corrosion rate at top of line
[Adapted from Marc et. al (2004)] ……………… ……………………………………….. ..12
Figure 3. Influence of temperature on corrosion rate at the bottom of line
[Adapted from Marc et. al (2004)] ………………………………………………………….…..13
Figure 4. Influence of the condensation rate on the corrosion rate at the top of line
[Adapted from Marc et. al (2004)] ………………………………………………………………14
Figure 5. Influence of the condensation rate on the corrosion rate at the bottom of line
[Adapted from Marc et. al (2004)] …………………………………………….………………..14
Figure 6. Test section of the experiment [Adapted from Zhang et. al (2007)] …………………15
Figure 7. Comparison of measured and predicted condensation rates
[Adapted from Zhang et. al (2007)] ……………………………………………………………..16
Figure 8. Comparison between the model and long term experiments
[Adapted from Zhang et. al (2007)] ……………………………………………………………..16
Figure 9. Comparison between the model and long term experiments
[Adapted from Zhang et. al (2007)]………………………………………..……………..……..17
Figure 10.Corrosions to be divided based on o’clock orientation of the pipe ……….…..….…18
Figure11. Cross section of pipe divided into top onof line (TLC) and
bottom of line (BLC) corrosions …………………………………………………………..……18
Figure 12. stem and leaf display for 2007 top of line …………………………..………………23
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Figure 13, Letter Value display for 2007 Top of Line …………………………………………24
Figure 14, 2007 data box plot …………………………………………………………………..24
4.1 Corrosion Characteristics Based on Different Region 22
4.1.1 Stem and Leaf for 2007 Data 22
4.1.2 Letter Value Display 22
TABLE OF CONTENTS
Contents Page
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4.1.3 Box Plot Display 24
4.1.4 O’clock Orientation Corrosion 25
4.2 Top of Line Corrosion Modeling 26
4.2.1 Historical Corrosion Development 26
4.2.2 Box Plot Comparison 27
4.2.3 Stem and Leaf Comparison 28
4.2.2 Median Polish Model 29
5. Conclusion and Recommendation 32
6. References 33
Contents Page
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CHAPTER 1
INTRODUCTION
1.1 Background
Offshore pipeline has been the most important and efficient oil and gas transportation
from the sources of generation. As the oil and gas flow inside the pipe, serious chemical
reactions occur between the pipe wall and oil and gas species leading to serious damages in the
pipe. This issue starts to be an upsetting matter for the oil and gas industries since the last few
years and a vital topic for the scientist which needs to be studied and solved.
Corrosion is the chemical or electrochemical reaction between a material, usually a
metal, and its environment that produces deterioration of the material and its properties (1).
Corrosion weakens the durability of pipes and in some cases leakage occurs resulting in a serious
environmental damaging to the surrounding and economic lost. In the usual pipes (water,
drainage, and wastewater pipes) mostly a hydration reaction occurs inside the pipe leading to
block the fluid from flowing or decrease the efficiency of the pipe, while in offshore pipes
hydration is neglected due to the existence of glycol and methanol as inhibitors playing a role of
preventing the hydrate on pipeline wall (2). Instead corrosion occurs in offshore pipes.
The corrosion occurs when the materials of the pipe tend to reform to its origin form or
state by reduction of electron, where iron in pipe react with water leading to reduction of the wall
thickness taking place mostly in top of line (TLC) due to the condensation of water from gas and
in the bottom of the pipe, lateral wall is side effected mostly due to the condensed water falling
across due to the gravity force (3).
This research focuses on the carbon dioxide ( ) internal corrosions observed at the
top of line of the pipe.
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1.1.1 Factors of Pipe Line Corrosion
The factors of corrosion are the primer stage for a wider understanding and analyzing
of corrosion in pipeline. The scientists draw a conclusion that the most effective way to control
corrosion is by minimizing and monitoring the factors of corrosion. The main factors which
affect the pipeline corrosion are:
1. Water chemistry
2. Protective scales formation at the steel surface slowing down the corrosion process (4)
3. PH value, when the PH is low the environment in internal pipe line is more to corrosive,
vice versa (5)
4. CO2 partial pressure increasing typically leads to an increase on corrosion (6)
5. The effect of HAc, acid leads to decrease pH value (7)
6. Temperature
7. Flow Velocity increasing can damage the protective scales formed
The dimension and shape of corrosion varies from top to bottom of line. The corrosion
rate at the bottom is an order of magnitude higher than the top of the line (7). Typically, it will
have an irregular depth profile and extend in irregular pattern in both longitudinal and
circumferential directions (8).Forms of corrosion can be classified as crevice, galvanic,
intergranular, velocity- or microbial-induced corrosions, or even stress corrosion cracking and
selective leaching as schematically(2). Discussion on the dimension of corrosion exceed the
scope of this paper, interested readers can refer to the work by(2)for more understanding on the
subject matter.
1.2 Problem Statement
The top of line corrosion (TLC) has higher proportion of concern than the bottom
corrosion due to the complexity of corrosion process and the difficulty of analyzing them as
there are various factors affecting the TLC corrosion. Literatures showed that the existing
corrosion modeling predictions for the TLC presented are entirely based on an empirical and
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simulations. Therefore this research proposes a simplified approach to predict the TLC corrosion
based on statistical approaches called the Exploratory Data Analysis (EDA) techniques.
1.3 Objectives
This research intends to:
1. Statistically analyze corrosion occurrence along the circumference of an offshore
pipeline.
2. Develop prediction models for the top-of-line corrosion in offshore pipeline using the
Exploratory Data Analysis (EDA) techniques.
3. Compare historical corrosion development in a pipeline based on the median polish
models.
1.4 Scope of Study
Analyzing the top of line corrosion (TLC) using Exploratory Data analysis (EDA)
techniques for historical corrosion development in offshore pipelines in Malaysia.
1.5 Relevance of Study
This research will focus mainly on top of line corrosion in pipe line. TLC starts to have
more attention in offshore industry due to the complexity of corrosion and the difficulty to
maintain the defects. The difficulty lies on how to protect the top side of pipe where injection of
inhibitors method is impossible because of low water existence. Hereby, a mathematical model
will be developed to predict TLC corrosion mainly in Malaysian offshore pipeline system.
1.6 Feasibility of Study
Gas leakage and piping maintenance is a growing concern for oil and gas companies due
to its high cost expenses. The importance of study in real life is to avoid leakage and oil spill
from pipeline. Leakage of pipeline materials severely affects the surrounding environment. The
model developed will mathematically predict the corrosion in pipelines and foresee failures
before occurrence.
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CHAPTER 2
LITERATURE REVIEW
This chapter provides a discussion on the literatures of corrosion and its governing
factors for better understanding on the subject matter. Majority of the literatures are carried out
based on the concern on the factors. A number of experiments established for seeking advance
information about the factors impact on pipe and how to inhibit or minimize the effect on the
pipe.
2.1 Factors Affecting Corrosion
2.1.1Protective Scales
Protective scales are precipitation of scales in the water drops or the water
concentration along the pipe wall. When the precipitation is high enough to create the protective
scales it attaches to the wall protecting the pipe wall from corrosion. The most common scale
takes its place in corrosion is iron carbonate.
The protective scale techniques to low the corrosion is by diffusing the species involved
in corrosion process or cover the wall pipe isolating it from the corrosion species.
Protective scales technique of protection is very efficient and appreciated but must be
considered that the formation of protective scales requires a special environment and treatment.
As mentioned previously the formation of protective scales needs a high precipitation of iron
species on the condensed water, but even at high super saturation of iron carbonate the
formation is failed if the temperature is low. Conversely, at a high temp usually above 60°C
very protective scales can be formed even if the precipitation is low (5).
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2.1.2 Temperature
The Institute for Corrosion and Multiphase Technology at Ohio University, Athens has
build, with a support of the company TOTAL, an experimental flow loop especially designed to
study of the TLC. The velocity and pressure were fixed and other parameters were changed
while the value and rate of corrosion was recorded. The influence of temperature in the
experiment was studied at a concentration of HAc of 172 ppm, under a critical condensation rate
and at two different temperatures (50 and 70 °C). The results are presented on Fig.2 and 3. Note
that at high temperature the protective scales fail to form if the condensation rate is high (9).
Figure 1. SEM image of a cross section of a steel specimen including an iron carbonate
scale. Exposed for 10 h at T=80°C, pH 6.6, PCO2 ¼0:54 bar,c Fe2þ ¼250 ppm, and v= 1 m/s
[Adapted from fromNesic and Lee (2003)]
Figure 2. Influnce of temperature on
corrosion rate at top
of line,gas velovity of 5 m/s, HAc of
171 ppm, critical
condensation rate
and constant CO2pressure.
[Adapted from
Marc et. al (2004)]
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2.1.3 Condensation Rate
Condensation is the accumulation of water on the pipe wall due to the differences of
temperature between internal and external section of the pipe with the temperature of the gas
flowing.
The wet gas condensed on the pipe wall, thus corrosion takes a place. The condensation rate is at
highest value at the starting point of the pipe, because the temperature difference is at the highest
level.
2.1.3.1Top of Line Corrosion (TLC)
Top of line corrosion (TLC) occurs when a wet gas transport throw the pipe internal
wall allowing the wet gas to condense on the internal pipe wall by the force of heat exchange
between the outside environment and inside environment temperatures and the effect can be seen
in Fig. 4.
Figure 3. Influence of temperature on corrosion rate at the bottom of
line, gas velovity 5 m/s, HAc 171 ppm, critical condensation rate and
constant CO2pressure. [Adapted from Marc et. al (2004)]
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Figure4.Influence of the condensation rate on the corrosion rate at the top of line, gas velovity 5 m/s, HAc 171 ppm,critical condensation rate and constant CO2 pressure. [Adapted from Marc et.
al (2004)]
2.1.3.2Bottom of LineCorrosion (BLC)
Due to the gravity force the water condensed at the top of line drives down through the
internal wall to the bottom of line (BLC).
The results in Fig.5 shows that the condensation rate does not influence the corrosion rate at the
bottom of line, the reason ilustrates due to the sagnificant amount of water presence on the
bottom of line.
Figure5.Influence of the condensation rate on the
corrosion rate at the
bottom of line, gas
velocity 5 m/s, HAc 171 ppm,critical
condensation rate,
constant CO2pressure, temp 70°C.[Adapted
from Marc et. al (2004)]
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2.2 Models on TLC Corrosion
1.2.1 Condensation Model
A mechanistic condensation model is established based on the corrosion growth rate.
All the calculation from this model was derived basically from the models developed by Nesic
and his research group [(1), (3)].Eventually, a verification of the model established by a
comparison between an experimental data and the predicted result from the model.
Fig. 6 shows the test section of the experiment and where the data is collected from. The test
section is equipped with a cooling system in order to control the inner temperature of the wall.
When a hot wet gas flow through the inner wall, the condensation occurs and then the condensed
water is drained downstream by a liquid collector.
The comparison between the experiments and model prediction are shown in Fig.7. The
model gives good and close results to the experiment work. The experimental results showed
close agreement with the predicted models.
Figure 6. Test section of the experiment
[Adapted from Zhang et. al (2007)]
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2.2.2 Corrosion Model
The corrosion model is concerned about the most important parameters and its effect
basically on top of line corrosion: gas temperature, partial pressure, velocity of gas,
condensation rate and HAc concentration. All are described by mathematical equationsin the
model, which can eventually predict the effect of corrosion on pipe line with time.
The results show a good prediction comparing with the experimental work done for long terms
rate prediction. For the short terms (2 days) the model over predicted the corrosion rate, the
researcher claims that the discrepancy for short term experiments probably results from the
introduced approximation of a 2D problem in a 1D approach (6).
Figure 7. Comparison of measured and predicted condensation rates [Adapted from Zhang et. al (2007)]
Figure 8.
Comparison
between the model
and long term experiments
(T=70°C, V= 5 m/s,
P = 3 bar, P = 2 bar, condensation
rate= 0.25
mL/ /s) [Adapted
from Zhang et. al (2007)]
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As a result, Fig.8 shows the comparison between the model and experiments in graph.
It can be seen that the corrosion rate at the beginning is very high, this due to the initial
condensed water which is very corrosive. The corrosion starts to reduce with time; this is
because of protective scales formation on metal surface. Over time, the scales becomes denser,
thus the corrosion decrease dramatically and remains at very low state in long terms.
Fig. 9 explains that the formation of scales still retards the corrosion even when the
temperature is 40°C which is consider low. The corrosion starts as very corrosive then gradually
decreased with the formation of scales. At the starting point the pH shows a low value, which is
not prober for the formation of scales. As the corrosion proceed and concentration of species
increase in the droplets the pH tends to increase in value, thus the scales form.
Figure 9. Comparison between the model and long term experiments
(T=40°C, V= 5 m/s, P = 3 bar, P = 2 bar, Condensation rate= 0.25
mL/ /s) [Adapted from Zhang et. al (2007)]
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CHAPTER 3
METHODOLOGY
3.1 Research Concept
The research intends to generate a model from sets of data collected from pipelines in
Malaysia. The set of data is analyzed statically by using a statistical technique called the
Exploratory Data Analysis (EDA). The EDA analysis will be carried out through simulation
using a Minitab software. The data collected represents internal corrosions along the pipeline
which located on the pipe wall.
In order to simplify the data input into the analysis, the pipe is divided into o`clock orientation,
measured from the cross section of the pipe, as shown in
Fig. 10.
Hereby, the defects occurred in the pipe wall will be statistically placed based on its
location, as shown in Fig. 11. The expected outcome of the software is a model equation
developed to predict the top of line corrosion (TLC) in offshore pipelines (i.e. Objective 2).
Lastly, the model is used to compare historical pipeline corrosion in Malaysia (i.e. Objective 3).
Figure 10.Corrosions to be divided based on
o’clock orientation of the pipe
Figure11. Cross section of pipe divided into top of
line (TLC) and bottom of line (BLC) corrosions
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3.2 EDA
(EDA) is an approach to analyzing data sets to summarize their main characteristics,
often with visual methods. EDA is for seeing what the data can tell us beyond the formal
modeling or hypothesis testing task.
In this research EDA technique is used for a statically analyzing the data related to pipeline.
The implementation of this technique in this research is estimation the corrosion statically
and predicts the probability of future corrosion.
3.2.1 Box plot
In descriptive statistics, a box plot a convenient way of graphically depicting groups of
numerical data through their quartiles. Box plots may also have lines extending vertically
from the boxes (whiskers) indicating variability outside the upper and lower quartiles.
Outliers may be plotted as individual points.
Box plots display differences between populations without making any assumptions of the
underlying statistical distribution. The spacing between the different parts of the box help
indicate the degree of dispersion (spread) and skeness in the data, and identify outliers.
3.2.2 Stem and Leaf
A stem-and-leaf display is a device for presenting quantitative data in a graphical
format, similar to a histogram, to assist in visualizing the shape of a distribution. They
evolved from Arthur Bowley's work in the early 1900s, and are useful tools in
exploratory data analysis. Stem-and-leaf displays retain the original data to at least two
significant digits, and put the data in order, thereby easing the move to order-based
(5) Nesic, S. (2007). Key issues related to modelling of internalcorrosion of oil and gas pipelines.
Institute for Corrosion and Multiphase Technology, Ohio University,.
(6) Ziru Zhang, DezraHinkson, Marc Singer, SrdjanNesic, "A Mechanistic Model of Top of the
Line Corrosion", CORROSION/2007, Paper No. 07556, (Houston, TX: NACE International,
2007).
(7) Marc Singer, Srdjan Nesic. (2004). Top of the line corrosion in presence of acetic acid and carbon
dioxide.
(8) FrédéricVitse, SrdjanNesic, Yves Gunaltun, Dominique Larrey de Torreben, Pierre Duchet-
Suchaux, "Mechanistic Model for the Prediction of Top-of-the-Line Corrosion Risk",
CORROSION/2003, Paper No. 03633, (Houston, TX: NACE International, 2003). (9) Rolf Nyborg and Arne Dugstad. (2007). Top of Line Corrosion and Water Condensation Rates in Wet
Gas Pipelines. Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway.
(10) M.shitan, Turaj. (2011).Exploratory Data Analysis. University putra Malaysia